In the case of the electronic watch, there is generally employed an electromagnetic motor having a bipolar permanent magnet operating in a stepping mode, the rotor effecting a rotation through 180.degree. for each step. If the watch includes a seconds hand for which stepping takes place at a frequency of 1 Hz, there is then required a reduction by a factor of 30 between the rotor of the motor and such hand. The rotation of the rotor through 180.degree. per step and the gearing system serving for the reduction bring about energy losses basically due to friction and generate a relatively substantial noise.
Next, if one wishes to obtain an almost continuous displacement of the seconds hand with a bioplar permanent magnet, it is necessary to increase the operating frequency of the motor and consequently the reduction down between the rotor of such motor and the hand. This inevitably leads to additional energy losses and modifications of the timekeeping movement, especially insofar as these concern the dimensioning of the driving gears.
On the other hand, a multipolar motor showing a sufficient number of pole pairs enables reducing the energy losses, indeed the number of transmission gears and it renders possible an almost continuous displacement of the associated mechanism without thereby requiring a greater reduction than that which is necessary in the case of a motor with a bipolar permanent magnet, as mentioned hereinbefore.
It will also be observed that a motor with a multipolar permanent magnet may enable a lessening of the driving noise.
There is known in particular from European patent EP 0 151 158 a two-phase electromagnetic motor with an axially magnetized multipolar permanent magnet. The stator of this motor is basically formed from four principal stator parts and two magnetic flux guidance legs, each bearing a winding and each coupling two of such stator parts. The two stator parts coupled respectively to the first and to the second winding are profiled in a manner such that they fit into one another and that, when the first stator part is superposed on the south poles located on one of the two halves of the multipolar permanent magnet, the second stator part is superposed over the north poles of the same half of the multipolar permanent magnet.
The magnetization of the multipolar permanent magnet being axial, a disc of soft ferro-magnetic material serving as a magnetic flux guide is secured, in a first variant, on the face of the permanent magnet opposed to that located facing the stator parts. In a second variant, such disc is mounted in a manner such that the permanent magnet of the rotor is located between it and the general stator plane.
This motor exhibits the following disadvantages:
the two corresponding principal stator parts fitted into one another require a delicate adjustment. Additionally, they are not appropriate for a high number of poles. Next, a relatively substantial gap must be provided between such two parts located in a common plane in view of the magnetic potential difference between such two parts when the corresponding winding is energized. PA1 In the first variant, the disc of soft ferromagnetic material secured on one of the faces of the permanent magnet of the rotor gives rise to magnetic short-circuits between the magnetic poles of the rotor permanent magnet. Additionally, the rotor permanent magnet being located in a plane other than the general stator plane, it is attracted by the stator itself. It is thus necessary to exert a force thereon in order to hold it in place, which causes friction lowering the yield of the motor. Finally, such disc increases the inertia of the rotor without increasing its useful volume. PA1 In the second variant, the problem of positioning the rotor permanent magnet and the increase in inertia are partially resolved, but at the cost of an increase of magnetic reluctance. Effectively, the magnetic fluxes circulating in one of the motor magnetic circuits then traverse four gaps stator - permanent magnet of the rotor. PA1 a stator comprising two magnetic flux guidance legs and a first principal stator part defining a first stator hole having a central region and a peripheral region surrounding said central region, the contour of said first stator hole defining within said peripheral region a first gapped circular crown located in a first stator plane and formed of castellations separated by slots; and PA1 a rotor having a rotation axis substantially perpendicular to said first stator plane and traversing said central region of said first stator hole, such rotor including an even number greater than two of bipolar permanent magnets each defining a pair of magnetic poles arranged on either side of a general rotor plane substantially perpendicular to said rotation axis so that the magnetic axis defined by each pair of magnetic poles has an orientation substantially parallel to said rotation axis and opposed to that of the magnetic axes defined by the adjacent pairs of magnetic poles, said pairs of magnetic poles being at least partially arranged facing at least a superposition portion of said castellations; PA1 first and second windings mounted respectively on said first and second magnetic flux guidance legs; such electromagnetic transducer being characterized in that said stator comprises a second principal stator part having a superposition portion arranged to face said pairs of magnetic poles and defining a second stator plane substantially parallel to said first stator plane, said superposition portions of said castellations and of said second principal stator part being respectively located on a first side and on a second side of said general stator plane, said first principal stator part defining first and second principal magnetic poles and said second principal stator part defining at least one third principal magnetic pole, said first and second magnetic flux guidance legs magnetically and respectively coupling said first and second principal magnetic poles with said second principal stator part.
There is also known from Swiss patent CH 656 990 another two-phase electromagnetic motor having a multipolar permanent magnet.
The stator of this motor is primarily formed by a stator part defining a stator hole and four stator poles. Two adjacent stator poles are coupled to one another by a leg on which is assembled an energization winding. In the same manner, the two remaining poles are coupled to one another by a second leg bearing a second energization winding. The magnetization of the magnetic pole pairs of the rotor permanent magnet is axial.
The permanent magnet of the rotor is located in a neighbouring but different plane from the stator plane. In order to enable magnetic coupling between the stator poles and the permanent magnet, secondary stator poles are provided formed by teeth arranged over about a quarter of the circumference of the stator hole and directed towards the center thereof.
Each secondary pole belonging to a common principal pole is angularly separated from such principal pole by an even number of rotor poles relative to the other secondary poles of such principal pole and by an odd number of rotor poles relative to the secondary poles of the second principal pole energized by the same winding, whilst being angularly separated relative to the secondary poles of the other two principal poles magnetically coupled to the other winding by a whole number plus one half of rotor poles.
This special configuration of the secondary poles enables operation at 60 steps per revolution for a permanent magnet possessing thirty pairs of axially arranged poles by alternately energizing the two windings. Additionally, such a motor operates in the two possible rotation senses of the rotor.
However, this motor exhibits at least two major disadvantages. Initially, in order to permit the return of the magnetic flux, a disc of low magnetic reluctance is applied against the face of the permanent magnet which is opposed to the face located facing the stator. In the same manner as in the case of the first variant of the preceding motor, this plate brings about magnetic short circuits and thus increases the leakage fields of the rotor permanent magnet. Additionally, such plate increases the inertia of the rotor. Next, the rotor permanent magnet is also placed in a non-equilibrium position within the stator, that is to say, in a position not corresponding to the minimum energy of the stator - rotor system. Thus, the rotor magnet is attracted by the stator parts located in a plane neighbouring that of the permanent magnet. In order to maintain the magnet in its position, it is necessary to exert a mechanical sustaining force bringing about friction forces lowering the yield of such a motor.
Secondly, the two poles associated with the same winding must be magnetically insulated by a relatively large gap in view of the magnetic potential difference between such two poles when the associated winding is energized. This diminishes the possible utilization of the magnetized volume of the rotor permanent magnet.
The purpose of the present invention is to overcome the disadvantages described hereinabove in providing a two-phase multipolar electromagnetic transducer in which the position of the rotor permanent magnet corresponds substantially to a minimum energy position which is a balanced position of the latter within the stator, this motor further exhibiting a compact configuration, a very good yield and a relatively low manufacturing cost.